1. Field of the Invention
The present invention relates to a power supply circuit protecting device and in particular to a device for protecting a power supply circuit which prevents parts of the power supply circuit from overheating or burning by stopping the oscillation of the power supply circuit when an abnormality such as short-circuiting between terminals of a power transformer and of a load circuit in the secondary side of the power supply circuit occurs and is excellent in safety.
2. Prior Art
Recently, switching type power sources have been predominantly used in consumer's electronic devices such as television receiver sets and simultaneously an improvement in the safety of the power sources has been demanded. Accordingly, various devices for protecting a switching power supply circuit in a television receiver, etc. have been proposed.
A prior art device for protecting a power supply circuit is shown in FIG. 2. A reference numeral 1 denotes an a.c. power inlet; 2 an a.c. fuse; 3 a full-wave rectifying circuit; 4 a power supply output control means; 5 a power transformer; 6 a photo-coupler for power supply voltage feedback control; 7 a voltage detection error amplifier; 8 an activating resistor; 9 an activating diode; 10 a current detecting resistor; 11 and 12 resistors; 13 through 16 rectifying diodes; 17 through 21 smoothing capacitors. The outputs of the voltage lines are connected to loads 100, 101, 102, respectively.
Operation of the thus formed prior art power supply protecting device will be described.
When a.c. voltage is applied to the power supply circuit, the power output control means is activated through the activating resistor 8 so that a voltage is output to the secondary side of the transformer 5. A change in voltage in the secondary side (140 voltage line) of the power transformer is detected by the voltage control error amplifier 7 and is converted into a change in current. The converted current change is fed back to a control terminal of the power supply output control means 4 to control an output voltage.
Operation when a circuit across the terminals S4 and S5 (10 V and the ground) of the power transformer 5 is shorted will now be described.
The winding between S4 and S5 is wound in turns of 2T. A short-circuiting current flows through the winding between S4 and S5 when the circuit between S4 and S5 is shorted so that the temperature of the transformer is elevated. Since an increase in current flowing through the primary winding at this time is relatively low (since the secondary winding is in turns of 2T), the power output control means 4 continues to normally work. If this condition continues for a long period of time, the temperature of the winding between S4 and S5 is considerably elevated and the winding is then brought into contact with the winding between S3 and S5. Since the current flowing through the primary side is considerably increased at this time, the a.c. fuse 2 is fused or a circuit between the drain and the source of a MOSFET in the power output control means 4 is opened so that the power supply circuit is turned off.
There remains a problem in reliability of the dielectric resistance across the primary and secondary windings of the power transformer since heat generation of the power transformer 5 continues for a long period of time. There is also a problem in that a stress is applied to the power output control means 4, causing a break-down thereof.